It is desirable to reversibly wet or de-wet a surface, because this allows one to reversibly control the mobility of a fluid on a surface. Controlling the mobility of a fluid on a surface is advantageous in microfluidics applications where it is desirable to repeatedly move a fluid to a designated location, immobilize the fluid and remobilize it again. It is also advantageous to control the mobility of a fluid on a surface of a body when moving the body through a fluid. Unfortunately existing surfaces do not provide the desired reversible control of wetting.
For instance, certain surfaces with raised features, such as posts or pins, may provide a superhydrophobic surface. That is, a droplet of liquid on a superhydrophobic surface will appear as a suspended drop having a contact angle of at least about 140 degrees. Applying a voltage between the surface and the droplet can cause the surface to become wetted, as indicated by the suspended drop having a contact angle of less than 90 degrees. This is further discussed in U.S. Patent Applications 2005/0039661 and 2004/0191127, which are incorporated by reference herein in their entirety. Unfortunately, the droplet may not return to its position on top of the structure and with a high contact angle when the voltage is then turned off.